The present invention relates to concrete poles, and, in particular, to a concrete filled metal pole with excellent flexural characteristics.
In the prior art, many concrete filled steel columns may be found, such as U.S. Pat. No. 4,783,940 “Sato” which is for a concrete filled steel tube column, and U.S. Pat. No. 4,018,055 “Le Clercq” which is for a concrete filled steel caisson. In some instances, such as in “Sato”, the column is pre-stressed so that, when the column is subjected to axial loading, most if not all, of the loading is taken up by the concrete, which is excellent in compression, and very little of the load is transmitted to the steel casing. In these instances, the steel casing is used as a convenient way to hold the concrete during manufacture, and projections, if any, extending from the steel casing to the concrete are used either to reinforce the concrete (not the steel pole) or to be able to apply a pre-stress to the column such that, after loading, the concrete column preferentially takes up the load (instead of the steel casing).
A column is fixed at least at two locations, generally at the bottom (fixed to the ground by burying, bolting, etc.) and at the top where it is connected to other building structures such as ceilings, beams, etc. Resistance to axial loading is critical, but resistance to flexural loading is unimportant because there is little, if any, flex loading given that the column is fixed at least at two locations. The concrete is used because it is excellent in compression (axial loading), and the outside shell is used mostly (if not exclusively) to support the concrete. Radially extending projections (such as in Sato) are used to enhance the binding between the concrete and the shell so one can apply the post-tensioning loads.
A pole, on the other hand, is fixed at only one location (typically bolted or buried to the ground at one end), and at the other end, a weight (like from a light fixture or wires hanging from the pole) results in a cantilevered load which imposes horizontal loads (flexural loads) which must be addressed.
One possible solution is to increase the thickness of the steel pole, adding significant cost. But one soon reaches a thickness where the pole can no longer be formed. Furthermore, along streets and roadways, the Americans with Disabilities Act (ADA) requires a minimum of 24 inches between any sidewalk and the curb. Any pole installed along these streets must typically fit into this limited space (typically 24″), and this thus becomes the maximum dimension of the pole at the base, further limiting the thickness of the steel pole.